The present invention generally relates to an organic electroluminescence device. More specifically, the present invention relates to an organic electroluminescence device having a layered structure and emitting white or substantially white light.
An organic electroluminescence device is a device which itself emits light, and has features such that it can emit light with wide viewing angle and high luminance and can be reduced in thickness, and therefore, application of the device to next-generation flat display or its flat light source has attracted attention. The method for achieving full-color display using the organic electroluminescence device may be roughly classified into the following three methods.
The first one is a method in which light emitting portions respectively for red (R), green (G), and blue (B) are formed on a flat surface, for example, by a resistance heating deposition method using a metal mask. In the manufacturing process for this display, RGB, i.e., three types of elements (subpixels) are individually formed on the same substrate and combined to form one pixel. Therefore, there is a need to prepare a deposition mask having a fine pattern for pixels and align it on the substrate with high accuracy. This can be problematic in that the productivity is low and the cost is high in the manufacturing process.
The second one is a method in which full-color display is made using an organic light-emitting layer which emits monochromatic light, for example, blue light, and a color changing layer, provided on the forward side as viewed in the light emitting direction, for changing the blue light to red or green light.
The third one is a method in which light emitted from an organic electroluminescence device emitting white light is divided into RGB using a color filter. In the method of obtaining arbitrary light by using the white-light emitting layer and the color filter in combination, there is no need to align a deposition mask to produce individual colors of light, and therefore the number of manufacturing steps may be reduced, making it possible to improve the productivity in the manufacturing process and suppress the cost.
Examples of methods for obtaining white light emission include: (1) a method using RGB light emitting layers in combination; (2) a method for allowing a single or a plurality of light emitting layers to emit light having two wavelengths which have the relationship of complementary colors, such as, bluish green and red light, or blue and yellow to orange light; and (3) a method utilizing exciplex emission. As the method (1) above, a method in which blue-, green-, and red-light emitting layers are stacked on one another to obtain white light has been proposed (see, Japanese Patent Application Publication No. 07-142169). The method (2) above includes a method in which two light-emitting layers for individual colors are stacked (see, Japanese Patent Application Publication No. 06-158038 and 07-65958), a method in which emission of two colors of light is obtained from a single light emitting layer (see, Japanese Patent Application Publication No. 09-208946), and the like. As the method (3) above, a white-light device using a boron-hydroxyphenylpyridine complex (see, Angew. Chem. Int. Ed. 2002, 41, No. 1) and the like have been reported.